Using dd to duplicate one hard disk partition to another one

Situation:
* 1 hard disk completely installed as I like it.
* 1 hard disk without format.
* 1 extern disk with just format.
* VERY IMPORTANT: All hard disks must be to same size.

To copy the hard disk, which is completely installed as I like it, to an extern empty disk, first boot with a rescue cd then run:
root@buzz:~# dd if=/dev/sda of=/dev/sdb bs=2G 2> dd.errors

Notice that dd.errors is a file where all errors will be redirected.if means “input file”, in that case is the hard disk that is completely installed as I like it.of means “output file”, in that case is the extern disk.bs means count of blocks at a time. In that case are blocks of 2G. It shouldn’t be more than the size ram.

Now, you should ran dd one more time, from extern disk to the disk without format. So boot with a recue cd again in order to access to disk without format.
root@buzz:~# dd if=/dev/sdb of=/dev/sda bs=2G 2> dd.errors